1. Field of the Invention
This invention relates to an auto focus camera in which the focus adjustment of a photo-taking lens is effected on the basis of range finding information recording an object to be photographed.
2. Related Background Art
A lens shutter camera having a stop and shutter has heretofore adopted an exposure control system called the triangular or trapezoidal program system. In this program system, the change in aperture opening accompanying the lapse of time presents a triangular or trapezoidal shape in a graph in which aperture value and time are plotted as ordinate and abscissa, respectively. A combination of an aperture value and a shutter speed for obtaining proper exposure is determined accordingly.
In many of the focus adjusting mechanisms of lens shutter cameras, the photo-taking lens is stepdriven. In such focus adjusting mechanism, the total focusing distance range from a nearest focusing distance to the farthest focusing distance is predivided into a plurality of object distance ranges in the direction of the optic axis of the photo-taking lens. The object distance range on which an object to be photographed exists is discriminated, and the photo-taking lens is driven to the set position (the distance step position) of the photo-taking lens corresponding to the center of the discriminated distance range. When the object to be photographed lies at a position corresponding to the center of the distance range, a sharp object image is obtained, but when the object to be photographed lies at a position distant from the center of the distance range, for example, at a distance corresponding to a border position with respect to a next distance range, the object image lacks sharpness and apparently the resolving power of the photo-taking lens is reduced.
In recent years, there has been a tendency toward using two focal length lenses or zoom lenses in lens shutter cameras, and occasions in which portrait photographing is effected when the phototaking lens is changed over to the telephoto side have been increasing. In such portrait photographing, it is required that the background of the main object to be photographed be blurred and the main object to be photographed be emphasized. Therefore, it is desirable that the aperture of the lens be as fully open as possible.
However, there is a contradiction, because in portrait photographing, it is desirable that an object be photographed with a large aperture opening to blur the background, but to obtain a sharp object image, it is desirable that the object be photographed with a small aperture opening.